Phosphatase proteins, particularly members of the protein tyrosine phosphatase subfamily, are a major target for drug action and development. Accordingly, it is valuable to the field of pharmaceutical development to identify and characterize previously unknown members of this subfamily of phosphatase proteins. The present invention advances the state of the art by providing a previously unidentified human phosphatase proteins that have homology to members of the protein tyrosine phosphatase subfamily.
Protein Phosphatase
Cellular signal transduction is a fundamental mechanism whereby external stimuli that regulate diverse cellular processes are relayed to the interior of cells. The biochemical pathways through which signals are transmitted within cells comprise a circuitry of directly or functionally connected interactive proteins. One of the key biochemical mechanisms of signal transduction involves the reversible phosphorylation of certain residues on proteins. The phosphorylation state of a protein may affect its conformation and/or enzymic activity as well as its cellular location. The phosphorylation state of a protein is modified through the reciprocal actions of protein phosphatases (PKs) and protein phosphatases (PPs) at various specific amino acid residues.
Protein phosphorylation is the ubiquitous strategy used to control the activities of eukaryotic cells. It is estimated that 10% of the proteins active in a typical mammalian cell are phosphorylated. The high-energy phosphate that confers activation and is transferred from adenosine triphosphate molecules to protein-by-protein phosphatases is subsequently removed from the protein-by-protein phosphatases. In this way, the phosphatases control most cellular signaling events that regulate cell growth and differentiation, cell-to-cell contacts, the cell cycle, and oncogenesis.
The protein phosphorylation/dephosphorylation cycle is one of the major regulatory mechanisms employed by eukaryotic cells to control cellular activities. It is estimated that more than 10% of the active proteins in a typical mammalian cell are phosphorylated. During protein phosphorylation/dephosphorylation, phosphate groups are transferred from adenosine triphosphate molecules to protein-by-protein phosphatases and are removed from the protein-by-protein phosphatases.
Protein phosphatases function in cellular signaling events that regulate cell growth and differentiation, cell-to-cell contacts, the cell cycle, and oncogenesis. Three protein phosphatase families have been identified as evolutionarily distinct. These include the serine/threonine phosphatases, the protein tyrosine phosphatases, and the acid/alkaline phosphatases (Carbonneau H. and Tonks N. K. (1992) Annu. Rev. Cell Biol. 8:463–93).
The serine/threonine phosphatases are either cytosolic or associated with a receptor. On the basis of their sensitivity to two thermostable proteins, inhibitors 1 and 2, and their divalent cation requirements, the serine/threonine phosphatases can be separated into four distinct groups, PP-I, PP-IIA, PP-IIB, and PP-IIC.
PP-I dephosphorylates many of the proteins phosphorylated by cylic AMP-dependent protein phosphatase and is therefore an important regulator of many cyclic AMP mediated, hormone responses in cells. PP-IIA has broad specificity for control of cell cycle, growth and proliferation, and DNA replication and is the main phosphatase responsible for reversing the phosphorylations of serine/threonine phosphatases. PP-IIB, or calcineurin (Cn), is a Ca.sup.+2-activated phosphatase; it is involved in the regulation of such diverse cellular functions as ion channel regulation, neuronal transmission, gene transcription, muscle glycogen metabolism, and lymphocyte activation.
PP-IIC is a Mg.sup.++-dependent phosphatase which participates in a wide variety of functions including regulating cyclic AMP-activated protein-phosphatase activity, Ca.sup.++-dependent signal transduction, tRNA splicing, and signal transmission related to heat shock responses. PP-IIC is a monomeric protein with a molecular mass of about 40–45 kDa. One .alpha. and several beta. isoforms of PP-IIC have been identified (Wenk, J. et al. (1992) FEBS Lett. 297: 135–138; Terasawa, T. et al. (1993) Arch. Biochem. Biophys. 307: 342–349; and Kato, S. et al. (1995) Arch. Biochem. Biophys. 318: 387–393).
The levels of protein phosphorylation required for normal cell growth and differentiation at any time are achieved through the coordinated action of PKs and PPS. Depending on the cellular context, these two types of enzymes may either antagonize or cooperate with each other during signal transduction. An imbalance between these enzymes may impair normal cell functions leading to metabolic disorders and cellular transformation.
For example, insulin binding to the insulin receptor, which is a PTK, triggers a variety of metabolic and growth promoting effects such as glucose transport, biosynthesis of glycogen and fats, DNA synthesis, cell division and differentiation. Diabetes mellitus, which is characterized by insufficient or a lack of insulin signal transduction, can be caused by any abnormality at any step along the insulin signaling pathway. (Olefsky, 1988, in “Cecil Textbook of Medicine,” 18th Ed., 2:1360–81).
It is also well known, for example, that the overexpression of PTKs, such as HER2, can play a decisive role in the development of cancer (Slamon et al., 1987, Science 235:77–82) and that antibodies capable of blocking the activity of this enzyme can abrogate tumor growth (Drebin et al., 1988, Oncogene 2:387–394). Blocking the signal transduction capability of tyrosine phosphatases such as Flk-1 and the PDGF receptor have been shown to block tumor growth in animal models (Millauer et al., 1994, Nature 367:577; Ueno et al., Science, 252:844–848).
Relatively less is known with respect to the direct role of phosphatases in signal transduction; PPs may play a role in human diseases. For example, ectopic expression of RPTP.alpha. produces a transformed phenotype in embryonic fibroblasts (Zheng et al., Nature 359:336–339), and overexpression of RPTP.alpha. in embryonal carcinoma cells causes the cells to differentiate into a cell type with neuronal phenotype (den Hertog et al., EMBO J. 12:3789–3798). The gene for human RPTP.gamma. has been localized to chromosome 3p21 which is a segment frequently altered in renal and small lung carcinoma. Mutations may occur in the extracellular segment of RPTP.gamma. which renders a RPTP that no longer respond to external signals (LaForgia et al., Wary et al., 1993, Cancer Res 52:478–482). Mutations in the gene encoding PTP1C (also known as HCP, SHP) are the cause of the moth-eaten phenotype in mice that suffer severe immunodeficiency, and systemic autoimmune disease accompanied by hyperproliferation of macrophages (Schultz et al., 1993, Cell 73:1445–1454). PTP1D (also known as Syp or PTP2C) has been shown to bind through SH2 domains to sites of phosphorylation in PDGFR, EGFR and insulin receptor substrate 1 (IRS-1). Reducing the activity of PTP1D by microinjection of anti-PTP1D antibody has been shown to block insulin or EGF-induced mitogenesis (Xiao et al., 1994, J Biol Chem 269:21244–21248).
The present invention has substantial similarity to protein tyrosine phosphatase (receptor type, Q). It is well established that protein tyrosine phosphorylation plays a key role in regulating structure proteins in migrating cells. Migrating cells interact with the extracellular matrix via focal adhesions (FA), which are contact points that link actin stress fibers to the membrane cytoskeleton and to transmembrane integrins. Engagement of integrins by the extracellular matrix in migrating cells induces tyrosine phosphorylation of several FA components including pp125FAK and paxillin. Specific PTPases have been linked to FA phosphorylation. For example LAR, a broadly expressed receptor PTPase, localizes to FAs in migrating cells but seems to be excluded from developing FAs at extending lamellopodia. This is consistent with a role of this receptor PTPase in FA disassembly by serving to dephosphorylate components that were activated initially by phosphorylation.
The potential importance of PTPases in the glomerulus has been underscored by the recent identification of GLEPP 1, a type III receptor-like PTPase (rPTPase), which is localized to the specialized foot processes of the podocyte. GLEPP1 has been proposed to play a role in the regulation of podocyte foot process structure and function. In support of this hypothesis, GLEPP1 protein levels are reduced in several types of human glomerular disease and in several animal models of glomerulonephritis.
Glomerular disease is initiated by a variety of factors, including immunologic, infectious, and toxic agents, as well as hemodynamic processes. A central pathological feature of many types of acute and progressive glomerular disease is injury of mesangial cells, which respond by proliferating as well as by secreting growth factors and extracellular matrix proteins. This contributes to resolution of glomerular damage but may also lead to fibrosis, which occurs in many chronic disease processes. The glomerular mesangial cell is a mesenchymally derived cell that shares properties with fibroblasts and smooth muscle cells and provides structural support to the glomerular tuft.
PTPases play as potential mediators of the mesangial cell response in glomerular disease, because PTPases expressed in the rat anti-Thy 1 model, wherein a new receptor rPTP-GMC 1, expressed by glomerular mesangial cells. rPTP-GMC 1 is highly restricted to the mesangial cell and that expression is acutely up-regulated in actively proliferating and migrating mesangial cells in the anti-Thy 1 model. rPTP-GMC1 is similar in structure to GLEPP 1 and may sense or regulate cell-cell or cell-matrix interactions to mediate glomerular repair. For a review, see Wright et al., J Biol Chem 1998 Sep 11;273(37):23929–37.
The discovery of a new human protein phosphatase and the polynucleotides encoding it satisfies a need in the art by providing new compositions that are useful in the diagnosis, prevention and treatment of biological processes associated with abnormal or unwanted protein phosphorylation.